Various types of propulsion devices are known for propelling watercraft through water environments. Such types of propulsive devices include types that push water, push air, and that are propelled by other means, such as by rocket. This invention pertains particularly to water pushing propulsion devices.
Various types of water pushing devices have clearly been proven successful and have been utilized for decades and even centuries. Presently, the most common of such devices in use are the exposed rotating propeller and the ducted rotating propeller (the later often being referred to as “jet” propulsion). In most circumstances, the exposed type of propeller is the most efficient and practical means for powering a watercraft. However, in some situations, such as in situations involving a watercraft such as a Jet Ski® or a Wave Runner®, an exposed propeller would present significant risk of personal injury, and therefore a ducted propeller is utilized in place thereof. Although generally less efficient, ducted propellers present significantly less risk of personal injury.
In other situations, such as in military applications in particular, it is desirable to minimize the noise or acoustic signature being emitted from the propulsive device of the watercraft. This is particularly the case with military submersibles, such as manned and unmanned submarines, since acoustic emissions are often the primary way such watercraft are detected. However, watercraft such as fishing boats could also benefit from propulsive devices having reduced acoustic emissions, due to the impact of such emissions on fish. Nonetheless, due to the rotational nature of both exposed propellers and ducted propellers, such propulsion devices inherently produce appreciable acoustic emissions. Furthermore, the turbulence produced by either type of propeller propulsion also produces detectable acoustic emissions. This is the case regardless of whether cavitation occurs, albeit cavitation substantially increases the acoustic emissions. Thus, an alternative means of propelling watercraft, and in particular submersible watercraft, in a manner producing substantially less acoustic emission is desired.
It is also known that the friction between the hull of a watercraft causes a reduction in the momentum of the liquid stream passing adjacent the hull, which results in drag. The region in which reduction in momentum occurs defines a boundary layer of the liquid stream and, in general, decreasing the boundary layer thickness results in a decrease in drag. To this end, numerous products such as special waxes and other techniques have been utilized in an effort to reduce the skin friction of watercraft hulls. While such products and techniques have proven to be successful, other solutions to reducing drag remain desirable.